This invention relates to improvements in fiber optic sensor systems, and in particular, to the provision of temperature measurement and compensation mechanisms, calibration systems, and additional improvements both in operating methodologies and design features.
Optical fiber sensing systems have found applications in various environments. For example, the measurement of intravascular blood pressure of human patients has been accomplished using equipment manufactured by the present assignee, FiberOptic Sensor Technologies, Inc. (FST) in which a diaphragm at the fiber sensing tip deforms in response to pressure differentials, and thus modulates through a reflection, a light signal sent through the fiber. Changes in the distance between the deformed diaphragm and the optical fiber end, and the diaphragm shape, modulate the amplitude of light that is reflected back into the optical fiber. Accordingly, the intensity of the returned light signal is related to pressure acting on the sensing tip.
Applicants have made numerous advancements in the technology of fiber optic sensing systems which are principally oriented toward pressure measurement. The present assignee, FST also owns U.S. Pat. Nos. 4,711,246; 4,787,396 and 4,924,870, all related to various improvements in fiber optic sensors, and which are hereby incorporated by reference. While the systems in accordance with these prior patents provide excellent performance for the intended applications, applicants are seeking to enhance the application environments which fiber optic pressure sensors may be used in.
One particularly demanding application for a pressure sensor is that of sensing within an internal combustion engine combustion chamber. There are presently numerous sensor approaches toward conducting such pressure measurement to provide real time measurement of combustion chamber pressure, which information can be used for controlling engine operating parameters such as spark timing, air/fuel ratio, exhaust gas recirculation (EGR), etc. to optimize engine performance. However, such an application is an extremely demanding one for a sensor. Extreme temperatures and temperature ranges would be encountered during use, with high accuracy and low cost required for such a product. Moreover, the combustion chamber environment exposes the sensor to intense electromagnetic fields, mechanical shock, and a corrosive atmosphere.
In assignees previously issued U.S. Pat. No. 4,924,870, a technique for compensating a fiber optic measuring system was described. That system is particularly adapted for compensating the pressure reading output of the device with respect to differences in outputs of the light sources used to inject light pulses into the fiber, fiber-to-fiber variations, and the bending effect (i.e. loss of signal resulting from curvature of the fiber). The previously described calibration scheme operates by using a reflective coating at the sensing tip end of the fiber which is reflective to a calibration light signal which is returned along the fiber, whereas the pressure measuring light signal is transmitted through the coating and is modulated by the pressure responsive diaphragm. That calibration scheme is capable of calibrating the pressure responsive light signals for all of the principal variables affecting response along the length of the optical fiber. That system is not, however, capable of compensating for parameters aside from pressure which affect the pressure measuring light signal beyond the filament end. Accordingly, although that "dual wavelength" scheme operates extremely well in environments where temperature ranges are low, and where single uses are contemplated, it has limitations in environments where extreme temperature variations and time dependent changes can be anticipated.
Temperature is the primary source of inaccuracies of high temperature fiber optic pressure sensors operated on the principal of a flexing diaphragm. Temperature fluctuations and extremes affect each of the four primary areas of these sensors including, the: sensing diaphragm; sensing tip; fiber optic link; and the associated opto-electronics and electronics. In particular, high temperature extremes, such as encountered in automotive engine combustion chambers at the sensing diaphragm, and somewhat lower temperature fluctuations at the sensing tip, are the two dominating sources of errors. Large temperature extremes change mechanical properties of the diaphragm, mostly Young's modulus, resulting in temperature dependent deflection. Temperature induced expansion of the elements of the sensing tip change the relative position of the fiber end and the diaphragm, and causes transmission changes through the fiber.
One feature of this invention is a technique for temperature compensation in diaphragm-based fiber optic pressure sensors. The technique is designed to eliminate, or significantly reduce, undesired temperature effects on the deflecting diaphragm, sensor tip, optical link, and electronic interface. An underlying principal of the compensation technique described in this specification is the simultaneous measurement of diaphragm deflection and sensing tip temperature, and real time software correction for the temperature effect.
In accordance with this invention, several methods for temperature measurement are described for enabling temperature compensation. In a first approach, a temperature sensing light signal is chosen to have a wavelength at near the cut-off wavelength (i.e. boundary between high reflectivity and high transmissivity) of a filter coating on the optical fiber end at the sensing tip. Since the cut-off wavelength of a multilayer dielectic film filter changes in accordance with temperature, the intensity of a reflected back signal for a temperature compensating light signal can be used as a measure of temperature. The wavelength can also overlap a region of a sharp peak in transmissivity which is often found in such filter coatings. Since the wavelength of such a peak will shift in response to temperature it also provides a convenient opportunity for temperature measurement.
In another technique for temperature compensation according to this invention, two light signals are injected into the optical fiber having different launching conditions. It has been found that certain transmission modes are affected by temperature in a differential manner for applicants' sensor systems. For example, a launching condition in which the injected light intensity is concentrated mostly along the outer surface of the fiber is attenuated in response to higher temperatures more so than a mode which is concentrated at the center of the fiber, which is only weakly coupled to the fiber outer surface.
Another facet of this invention is a technique for reducing the distortion effect of temperature variation and other factors beyond the optical filament end which does not require actually measuring temperature. This technique is used with dual wavelength systems as described in assignees U. S. Pat. No. 4,924,870 and involves allowing a calibration light signal to be partially transmitted and partially reflected by the filament end filter coating. This approach can be shown to reduce the span of errors which are not directly compensated for through the dual wavelength system.
In addition to the temperature effects mentioned previously, applications in which the same sensor is expected to provide measurements over a period of time gives rise to concern over time dependent changes in the sensing tip. For example, the reflectivity of the deformable diaphragm caused by oxidation or other effects can dramatically alter the intensity of the returned light signal, and hence, affect measurement. In accordance with this invention, a calibration scheme based on zero-setting the system when it is exposed to a known pressure is provided which is specifically oriented to an automotive application.
This specification further describes a temperature shielding feature for fiber optic pressure sensor tips exposed to high temperatures, such as those found in internal combustion engines. By reducing the range of temperatures to which the sensor tip is exposed, some of the temperature dependent effects can be reduced.
In applications where multiple pressure sensors are used, this invention further contemplates techniques for multiplexing a number of identical sensors performing simultaneous pressure measurement.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.